It is very often desirable to transfer a fluid, whose temperature differs from that of the ambient, from one location to another with a minimum of heat exchange between the fluid and the ambient.
For instance, warm petroleum gas and/or liquid may have to be pumped through pipes situated in arctic conditions. An excessive reduction of the fluids' temperature causes an increase of its viscosity to such an extent that it becomes difficult or impossible to pump economically. There is also the risk that if the pipe passes through permafrost zones these zones can be melted with subsequent loss of their mechanical support function.
District heating employs the transportation of heated water from a central source, such as a power station or geothermal area, to a whole community of users which may be situated at some distance from the source.
Chemical processing often requires the transport of fluids at very high temperatures during their transformation into an end product.
The economics of these processes depend, among other factors, on the extent to which heat losses can be prevented or at least minimized.
Another case in which heat exchange between a fluid being transported in a pipeline and the ambient must be reduced as much as possible is in the transport of cryogenic or liquified gases. Such cryogenic liquids may be being used for instance to cool superconducting electrical power lines or magnets or may even serve as a fuel in rocket propulsion.
Whatever the nature or purpose of the fluid, some form of thermal insulation must be provided around the pipe which transports the fluid.
Layers of thermally insulating material, such as foamed plastic, asbestos or glass fibre, have been proposed, but they can easily become impregnated with moisture and lose their insulating properties. Their mechanical resistance is very poor, and they can very easily be damaged during transport or rough handling.
It has been proposed that vacuum be used as an insulating medium. This can be accomplished by the use of two concentric tubes in which the inner tube is used to transport the fluid and the jacket space between the inner and outer tubes is evacuated. See, for instance, U.S. Pat. No. 1,140,633. While the use of vacuum insulation has proved very effective in considerably reducing heat exchange between the transported fluid and the external ambient, it has proved difficult to maintain the vacuum at a sufficiently low pressure to maintain the integrity of the insulation for a sufficiently long time. If the evacuated volume has a residual gas pressure of greater than about 10.sup.-4 torr (1.3.times.10.sup.-2 Pa)-10.sup.-3 torr (1.3.times.10.sup.-1 Pa), the thermal exchange between the inner and outer walls due to conduction and convection becomes significant. Continuous outgassing from the surfaces in contact with the vacuum causes the pressure to rise and eventually destroy the insulating properties of the structure. In theory, it would be possible to re-evacuate the space at intervals of time, but this involves additional labour costs and very often the pipelines are situated in remote and inaccessible locations, such as arctic tundra, or they are buried underground.
It has been proposed to place zeolites within the vacuum space as, for instance, in U.S. Pat. No. 3,369,826, in an attempt to maintain the required degree of vacuum, as it is well known that zeolites are capable of sorbing gas. Unfortunately, zeolites must be cooled to cryogenic temperatures in order for them to have an appreciable gas sorbing action. In addition, water vapour is preferentially sorbed by zeolites, which prematurely inhibits their sorption of other gases. Upon heating from the cryogenic temperatures, the sorbed gases are released.
Charcoal and PdO have also been suggested in U.S. Pat. No. 3,992,169, but PdO is capable of sorbing H.sub.2 only. Furthermore, palladium is expensive.
It is, therefore, necessary to provide some means for continuously maintaining a vacuum within the inter-pipe space for long periods of time without the necessity of human intervention or the use of automatic, and expensive, additional equipment.
Care must also be tkaen that any heating of the tubes does not cause metallurgical phase changes which would alter their physical or mechanical properties.